Laser Jello

Use gelatin as a smoked lens, to view total internal
reflection, and as a color filter.

D-shaped refraction or petri
dishes

One red-colored and one
blueberry package or Jello

A laser (preferably HeNe)

Make the gelatin according to the Jigglers recipe.
Then, shine the laser through it.

#1 Shine the laser through the red gelatin first.
Notice that a beautifully visible beam travels through it.
Then shine the laser through the blueberry gelatin.

#2 Using the red gelatin and the curved side of
the dish, shine the laser parallel to the central axis. Move
the laser from the center to the outside edge. Notice the
change and measure the angle of refraction.

#3 Turn the dish around. Shine the laser
perpendicular to the staight edge. Move the laser to the
outside edge so that the beam reflects off the surface of
the gelatin and is re-emitted parallel to the incoming beam.
(See title photo) Repeat the activity, bouncing the beam off
the straight side, to measure the criticle angle.

#4 If you have two lasers, align them parallel and
shine them through the curved side of the dish. Use a white
piece of paper or waxed paper as a screen to find the focal
point. Invert the gelatin lens onto a piece of waxed
paper--it may need a little prying to get it to come out.
Use the empty dish as a cookie cutter to make a concave lens
out of the original convex gelatin lens. Shine parallel
lasers through the new lens.

#1 Red objects appear red because they don't
resonate at red frequencies. The red wavelength energy is
either reflected or transmitted. Gelatin is colloidal and
scatters enough of the beam to make it visible. The
blueberry Jello is actually cyan, a mixture of blue
and green (check it on the overhead with a spectrum created
by a diffraction grating). Cyan is the complementary color
of red. The blueberry color transmits green and blue, but
changes red wavelengths into heat.

#2 As light enters the gelatin, the change in
medium causes a change in speed. A ray of light will refract
similar to the way a car will change direction as it moves
at an angle from asphalt to soft dirt or loose gravel.

#3 As light tries to travel from a slower (or more
optically dense) substance to a faster medium, it may
reflect similar to the skimming of a stone off the surface
of water. If the beam hits at an angle that is small
relative to the surface , then the light will reflect; if
the angle is closer to perpendicular, then the beam will
refract.

#4 Light travelling through the convex lens will
converge. Light travelling through the concave lens will
diverge. This might be a good time to introduce the
reversibility of light. Shine the light through the jello lens. Mark its path into and then out of the jello. Then shine a laser backwards along this path. That is, shine it into the path that the original beam exited. The light path followed by the reversed beam will be exactly the same.